Commutator, brushes; start shunt generator no load

Function of the commutator

• It acts as a mechanical rectifier: it converts the alternating emf induced in the armature conductors into a unidirectional (DC) voltage at the brushes.
• It collects the current from the armature conductors and passes it to the brushes.
• It connects the correct coil to the brush at the correct time. The copper segments are separated by mica insulation.

Function of the carbon brushes

• They make sliding contact with the commutator and carry the armature current to the external load.
• Carbon is self-lubricating and causes little wear on the commutator, and its high contact resistance reduces sparking.
• They are placed at the magnetic neutral axis for sparkless commutation.

Why a shunt generator is started without load

A shunt generator is self-excited. The emf builds up from the residual magnetism like this: residual emf $\to$ small field current $\to$ more flux $\to$ more emf, until the field-resistance line meets the magnetisation curve.

If the load is connected during starting:

1. The terminal voltage is very low, so a large part of the armature current goes to the load and the field current is reduced ($I_{sh} = V/R_{sh}$), because the load is in parallel with the field.
2. The voltage drop $I_aR_a$ in the armature takes away a larger share of the small emf, so the terminal voltage stays near zero.
3. The armature reaction of the load current weakens the small flux further, so the machine may fail to build up its voltage at all.

Hence the load is connected only after the voltage has reached its rated value.